US7153401B2 - Current-biased potentiometric NOx sensor for vehicle emissions - Google Patents
Current-biased potentiometric NOx sensor for vehicle emissions Download PDFInfo
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- US7153401B2 US7153401B2 US10/378,578 US37857803A US7153401B2 US 7153401 B2 US7153401 B2 US 7153401B2 US 37857803 A US37857803 A US 37857803A US 7153401 B2 US7153401 B2 US 7153401B2
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
- G01N33/0004—Gaseous mixtures, e.g. polluted air
- G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
- G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
- G01N33/0036—Specially adapted to detect a particular component
- G01N33/0037—Specially adapted to detect a particular component for NOx
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
- Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
Definitions
- the present invention relates to a NO x sensor system and more particularly to a current-biased potentiometric (chronopotentiometric) NO x sensor system for vehicle emissions.
- EP 0 Patent No. 769,694 published Apr. 23, 1997 for NO x sensor and method of measuring NO x issued to NGK Insulators, LTD invented by Nobuhide Kato et al. provides the following background information, “Various measuring methods and devices have been proposed for determining the concentration of NO x in a measurement gas.
- a known method employs a sensor comprising a Pt electrode and an Rh electrode formed on an oxygen ion-conductive solid electrolyte such as zirconia . . .
- Disclosed is a NO x sensor and a method of measuring NO x capable of obtaining a large change in signal to measure a concentration of low concentration NO x in a measurement gas continuously and accurately with good response over a long period of time.
- a NO x sensor comprises a first internal space into which the measurement gas is introduced through a first diffusion rate-determining passage, a second internal space arranged with a NO x -reducing catalyst, into which an atmosphere is introduced through a second diffusion rate-determining passage, an electrochemical pumping cell for controlling a partial pressure of oxygen in the internal space by using a first oxygen ion-conductive solid electrolyte and electrochemical cells provided in contact therewith, a partial oxygen pressure-detecting means for detecting the partial pressure of oxygen in the internal space by using the first oxygen ion-conductive solid electrolyte and electro-chemical cells provided in contact therewith, a first electrochemical sensor cell for outputting an electromotive force corresponding to the partial pressure of oxygen in the internal space, and a voltage-detecting means for detecting the electromotive force outputted from the first electrochemical sensor cell.
- the NO x concentration is determined from a value of the electromotive force of the first electrochemical sensor cell detected by the voltage-detecting means.”
- the present invention provides a nitrogen oxide sensor system for measuring the amount of nitrogen oxide (NO, NO 2 ) in a gas.
- the system comprises a first electrode exposed to the gas, an electrolyte positioned in contact with the first electrode, a second electrode positioned in contact with the electrolyte, and means for applying a fixed current between the first electrode and the second electrode (current-bias), and a means for monitoring the voltage across the two electrodes, which provides a measurement of the amount of nitrogen oxide in the gas.
- the current-biased technique (chronopotentiometric) is analogous to, but has distinct advantages over, voltage biased (amperometric) techniques reported in the literature.
- the first electrode and the second electrode are both exposed to the test gas.
- the first electrode is exposed to the test gas and the second electrode is exposed to a reference gas.
- the present invention also provides method of producing electrodes on a nitrogen oxide sensor system for measuring the amount of nitrogen oxide in a gas.
- the method comprises the steps of preparing a powder suspension of a metal oxide powder and dispensing by colloidal spray deposition the powder onto an electrolyte substrate to form an electrode.
- the present invention also provides method of producing a catalytically active composite electrode on a nitrogen oxide sensor system for measuring the amount of nitrogen oxide in a gas.
- the method comprises the steps of preparing a powder suspension of a metal oxide powder and an organometallic precursor and dispensing by colloidal spray deposition the powder onto an electrolyte substrate to form a sensing electrode.
- FIG. 1 schematically illustrates a sensor system having electrodes exposed to a gas and the use of a current bias between the electrodes.
- FIG. 2 is a graph of the current-voltage characteristic of the sensor in 10% O 2 and 10% O 2 +500 ppm NO at 650° C.
- FIG. 3 is a graph showing the sensor response versus time in 10% O 2 and 10% O 2 +500 ppm NO at 650° C.
- FIG. 4 is a graph of the logarithmic response of the sensor as a function of NO concentration in 10% O 2 at 650° C.
- FIG. 5 schematically illustrates another embodiment of a sensor system with both electrodes being on the same side of the electrolyte.
- the sensor system 100 provides an illustration of one embodiment of the present invention.
- the illustration is highly simplified in order to explain the embodiment of the invention.
- all the amperometric designs discussed in the literature go to great lengths to establish a constant O 2 concentration in at the sensing electrode. This is done by the use of ‘internal spaces’ separated by ‘rate limiting passages’.
- sensor temperature is generally closely controlled by the use of an integrated resistive heater. These details are not shown in FIG. 1 .
- the sensor system 100 comprises a number of elements including a first sensing electrode 102 , an electrolyte 101 , a second reference electrode 103 , a conduit for the gas 105 , and the gas 106 .
- These components are known in the prior art, for example see U.S. Pat. No. 5,554,269 issued Sep. 10, 1996 to the Gas Research Institute, which is incorporated herein by reference.
- U.S. Pat. No. 5,554,269 shows a sensor system for accurately measuring nitrogen oxide (NO x ) in a gas mixture via the use of at least one electrochemical sensing cell and differential pulse voltammetry (DPV).
- DPV differential pulse voltammetry
- the sensor system has a sensor with an electrochemical sensing cell for producing an electrical signal (current, voltage, etc.) indicative of an amount of the nitrogen oxide within the gas mixture.
- the sensing cell has an electrolyte interposed between an anode electrocatalyst and a cathode electrocatalyst.
- the sensor systems 100 includes a control unit 104 connected to electrode 103 by the connector 107 and connected electrode 102 by the connector 108 .
- the control unit 104 includes a means for applying a fixed current between said first electrode 102 and said second electrode 103 and for monitoring the voltage required to maintain the fixed current to provide a measurement of said amount of nitrogen oxide in said gas. This voltage changes as a function of the gas composition.
- the use of a voltage measurement enhanced by a fixed-current bias is a novel configuration for a high temperature electrochemical NO x sensor.
- the electrodes 102 and 103 are both exposed to the gas 106 .
- the additional embodiment of a sensor system having the first electrode 102 exposed to a gas to be measured and the second electrode 103 exposed to a reference gas is illustrated by the additional embodiment components 106 A and 110 A.
- the additional embodiment component 106 A illustrates the reference gas 106 A.
- the additional embodiment component 110 A illustrates a partition 110 A that separates the conduit 105 into two enclosures, a first enclosure for the test gas 106 and a second enclosure for the reference gas 106 A.
- the sensor system 500 comprises a number of elements including an electrolyte 501 , a first sensing electrode 502 , a second reference electrode 503 , a conduit for the gas 505 , and the gas 506 .
- the electrodes 502 and 503 are both on the same side of the electrolyte 501 .
- a control unit 504 is connected to electrode 503 by the connector 507 and is connected electrode 502 by the connector 508 .
- the control unit 504 includes means for applying a fixed current between said first electrode 502 and said second electrode 503 and means for monitoring the voltage required to maintain the fixed current to provide a measurement of said amount of nitrogen oxide in said gas.
- the electrical measuring equipment is known in the art.
- the means for applying fixed current between the first electrode and the second electrode and monitoring voltage to provide a measurement of the amount of nitrogen oxide in the gas provides a constant current (called ‘chronopotentiometric’) measurement.
- the electrical measuring equipment is known in the art.
- the current-biased mode results in a potentially higher signal to noise ratio than the amperometric mode, and a much higher sensitivity and faster response than the potentiometric mode.
- the current-biased mode retains the logarithmic response characteristic generally associated with the potentiometric mode, while maintaining the speed of response generally associated with the amperometric mode.
- the logarithmic response is generally considered preferable since it provides a wider dynamic range and better sensitivity at low NO x concentrations. Supporting data are presented below.
- the current-voltage characteristic of the sensor is shown in 10% O 2 at 650° C. with and without 500 ppm NO present in the gas. It can be seen that at certain fixed current levels, for example ⁇ 22 microamps in the FIG. 2 , the associated voltage changes significantly with the introduction of NO to the gas. This is in contrast to the traditional mode of amperometric operation, where a constant voltage is applied and the resultant current is measured, or the potentiometric mode, where the voltage is measured in the absence of any applied bias.
- FIG. 3 a graph is provided where the sensor response at 650° C. is shown when 500 ppm NO is introduced to flowing 10% O 2 , balance N 2 , and the sensor is operated with a constant current bias.
- both electrodes are exposed to the test gas, and the electrodes are located on opposite sides of the electrolyte, as in FIG. 1 .
- the sensor response has excellent signal-to-noise ratio ( ⁇ 50) and fast response (90% recovery of the baseline is ⁇ 1.5 seconds). Applicants are not aware of any published data showing faster response with comparable amplitude at any temperature.
- FIG. 4 a graph is provided where the sensor response at 650° C. is shown when varying levels of NO are introduced to flowing 10% O 2 , balance N 2 , and the sensor is operated with a constant current bias.
- both electrodes are exposed to the test gas, and the electrodes are located on opposite sides of the electrolyte, as in FIG. 1 .
- the sensor response is proportional to the logarithm of the NO concentration in the test gas. This is in marked contrast to the linear response generally attributed to traditional amperometric designs.
- the sensing electrode 102 is comprised of ceramic tin-doped indium oxide called ITO (indium tin oxide).
- ITO indium tin oxide
- the electrode is deposited on the surface of the electrolyte by a colloidal spray deposition of a nanocrystalline powder of the ITO. This is accomplished by mixing the ceramic powder and a suitable dispersing agent in a suitable solvent, and using an air-spray gun or an ultrasonic nebulizer to deposit the colloid onto the surface of the electrolyte.
- the electrolyte can be heated to assist the evaporation of the solvent.
- ITO for a sensing electrode in an electrochemical NO x sensor has not been reported in the open literature.
- ITO gives response characteristics as good or better than any reported to date in the open literature when used in t he current-biased configuration described above.
- Either indium-doped tin oxide or tin-doped indium oxide can be used. However, the tin-doped indium oxide is much easier to be coated as the electrode.
- Other doping elements such as antimony can also be used. While these materials are stated specifically here, the current-biased sensor is not specific to these materials, and other metal or metal oxide electrodes can be used.
- other deposition techniques besides colloidal spraying can be used to deposit the electrodes 102 and 103 onto the electrolyte 109 .
- the sensing electrode 102 comprises tin-doped indium oxide (ITO). In another embodiment the sensing electrode 102 comprises indium or antimony doped tin oxide. In another embodiment the sensing electrode 102 comprises a composite electrode composed of one of tin-doped indium oxide (ITO) or indium or antimony doped tin oxide combined with precious metal. In another embodiment the sensing electrode 102 comprises a spray deposited electrode. In one embodiment the spray deposited electrode comprises a metal-based organometallic precursor combined with a ceramic powder.
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Cited By (1)
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US9164080B2 (en) | 2012-06-11 | 2015-10-20 | Ohio State Innovation Foundation | System and method for sensing NO |
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US7828956B2 (en) * | 2006-01-09 | 2010-11-09 | Ford Global Technologies, Llc | Method for measuring concentrations of gas moieties in a gas mixture |
CN100412535C (en) * | 2006-04-21 | 2008-08-20 | 湖南大学 | Prepn process of ITO nanometer line and its gas sensor |
CN105424764A (en) * | 2015-11-05 | 2016-03-23 | 吉林大学 | Nitrogen dioxide sensor based on orderly-channel Ni-doped mesoporous indium oxide and preparation method thereof |
Citations (18)
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EP0257842B1 (en) | 1986-08-04 | 1992-11-11 | Ngk Insulators, Ltd. | Electrochemical nox sensor |
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Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US9164080B2 (en) | 2012-06-11 | 2015-10-20 | Ohio State Innovation Foundation | System and method for sensing NO |
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